Robotic-OCT guided inspection and microsurgery of monolithic storage devices

Data recovery from monolithic storage devices (MSDs) is in high demand for legal or business purposes. However, the conventional data recovery methods are destructive, complicated, and time-consuming. We develop a robotic-arm-assisted optical coherence tomography (robotic-OCT) for non-destructive inspection of MSDs, offering ~7 μm lateral resolution, ~4 μm axial resolution and an adjustable field-of-view to accommodate various MSD sizes. Using a continuous scanning strategy, robotic-OCT achieves automated volumetric imaging of a micro-SD card in ~37 seconds, significantly faster than the traditional stop-and-stare scanning that typically takes tens of minutes. We also demonstrate the robotic-OCT-guided laser ablation as a microsurgical tool for targeted area removal with precision of ±10 μm and accuracy of ~50 μm, eliminating the need to remove the entire insulating layer and operator intervention, thus greatly improving the data recovery efficiency. This work has diverse potential applications in digital forensics, failure analysis, materials testing, and quality control.


Introduction
The Introduction section provides a comprehensive overview of the challenges and limitations associated with current methods of data recovery from monolithic storage devices (MSDs).It introduces optical coherence tomography (OCT) as a potential solution and highlights its advantages over X-ray radiography.The integration of a robotic arm and the use of continuous scanning are also mentioned as important components of the proposed approach.Overall, the Introduction sets the stage for the study and establishes the motivation for developing the robotic-OCT system.It is a high-quality introduction that deserves praise.However, here are a few suggestions to further improve the Introduction: 1. Address potential concerns: Given that OCT is traditionally used in the biomedical field, it may be beneficial to briefly address any potential concerns or challenges in applying OCT to nonbiomedical applications such as MSD inspection.This could include factors like sample size, surface reflectivity, or other limitations that might affect the imaging quality or feasibility of the approach.
2.Clarify the novelty: Although the introduction mentions that this study is a significant improvement over traditional methods, explicitly emphasizing the novelty of this study can accurately highlight the research value more and contribution of the article and enhance the reader's awareness of the novelty and importance of the article.For example, the need and advantages of combining robotics with optical coherence tomography are emphasized.

Methods
The Methods section in this article is well-organized, detailed, and provides a comprehensive overview of the experimental procedures and techniques used to develop the custom-built robotic-OCT system.However, here are a few suggestions to further improve the Introduction: 1.It would be helpful to include more information about the specific components and manufacturers of the OCT engine, such as the brand and model of the super luminescent diode (SLD) and the k-linear spectrometer, OCT probe objective lens size, focal length, working distance.This additional detail will provide a clearer understanding of the setup and equipment used in the study.2.It would be valuable to mention any safety measures or precautions taken during the laser ablation microsurgery procedure.This could include details about laser power settings, safety protocols, and any measures implemented to prevent damage to the sample or ensure operator safety.3.Some graphs, such as Fig. 6 lack scale bars.

Result
The results section clearly explains the details of the customized robotic-OCT system and the experimental procedure.Here are a few suggestions to further improve the Introduction: 1.Clarify the specific benefits and advantages of the continuous scanning strategy.How does this strategy improve the imaging process compared to other scanning methods?Highlighting these advantages and discussing any limitations or trade-offs would provide a more comprehensive evaluation of the approach.2.Address the feasibility of extending the continuous scanning strategy to larger or irregularly shaped objects.Are there any practical limitations or challenges that need to be considered?Discussing the scalability and adaptability of the approach would contribute to the broader applicability of the robotic-OCT system.3.Additionally, it would be valuable to discuss the potential limitations or challenges encountered during the microsurgery procedure, such as the precision of pin exposure or any thermal effects caused by laser ablation.

Discussion
The authors' discussion highlights the novelty and potential impact of their work on the field of data recovery from microsurgery for MSDs.They provide a clear contrast between the traditional methods of data recovery, which are time-consuming, require expertise, and can cause damage to the samples, and the proposed robotic-OCT system, which enables automated, non-destructive, and fast imaging.The authors rightly point out that their work has the potential to revolutionize data recovery procedures for MSDs.
Reviewer #3 (Remarks to the Author): The authors do a good job introducing the optical coherence tomography as a safe and fast option for inspecting the internal PCB layers of the monolith flash storage devices which can substitute the traditional X-ray inspection.The authors also automated the whole process which can help make forensic data extraction more efficient than before.This research can be a good reference for digital forensics labs to introduce OCT instead of X-ray for reverse-engineering the target device PCBs.Also, this work can contribute to make the data acquisition process quicker than before in digital forensics.However, one thing I feel missing is the detailed evaluation of the OCT inspection.The authors do mention the limitation saying that the 2nd layer of the PCB cannot be observed due to the thickness of the epoxy.But up to how many micro meters can the OCT penetrate to observe further layers?If there are more than 2 layers of PCBs, then how many layers can be inspected?And does the material of the epoxy affect the efficiency of OCT inspection?Those details should be discussed in the article.The authors suggest using the mid-infrared OCT, but no further detail is discussed.Please make it clear what becomes better and what limitation the operators would face by changing the wavelength of the infrared.Overall, the article lacks detailed explanation of each steps and investigations.For example, in section 2.5, the authors inspect variable damaged SD cards, but its results are pretty vague.For example, for the scratched card, how deep is the scratch based on the OCT inspection?Is it reaching the flash die and the data is not recoverable?Other sections are also a bit too much summarized.Please try to be more descriptive.The work itself is nice and helpful for forensic investigation.Since applying OCT in device inspection is the main part of this work (in addition to automating the laser ablation process), the readers would expect more evaluation details.Also please reconsider the structure of the article.The whole time I was missing the background information (the one mentioned in section 4) and it was hard to follow all the sections.

Point-by-QPKOT RGSQPOSG TP TJG >GVKGWGRS] EPNNGOTS
We thank the three Reviewers for their valuable comments and suggestions.We have revised the manuscript and provided new results to address all the concerns raised.>7A97B7> 5<::7;@?>GVKGWGR $+ %>GNCRLS TP TJG 4UTJPR&3 In this study, the authors developed an OCT system for the inspection and microsurgery of monolithic storage devices.To examine such devices, conventional OCT does not provide a sufficient field of view.To address the unmet need, the authors attached the OCT scanner to a robotics arm and assemble macroscopic images.This manuscript presented non-invasive inspection of MSDs with high-quality OCT images.The work presented by this manuscript targeted a very specific application and the authors provided strong data to demonstrate the usefulness of the imaging platform.
Response: We appreciate the reviewer's acknowledgment of the significance of our work in addressing the unmet need in this field and the strong data presented in our manuscript.Please find below the answers to the specific questions.
1) The methodology is sound, and the conclusions are supported by strong experimental data.However, this study has limited novelty.

>GSQPOSG3
We sincerely appreciate the reviewer's comments and would like to address the concerns regarding the novelty of our work.We would like to highlight the novel contributions of our study as follows.
Novel contribution 1: While it is true that galvo-based OCT scanners have been previously attached to robotic arms or translation stages to extend the field of view (FOV) [22][31-33], they typically employ a stop-and-stare scanning approach, which is time-consuming and inefficient due to the frequent restarts of the robotic arm or translation stage, as well as the subsequent stitching of data from each scanned block.Unlike the conventional stopand-stare scanning approach, our study introduces a novel continuous scanning strategy specifically designed for the robotic-OCT system, where the robotic arm moves continuously over the object and captures data.By eliminating the need for start-stop motions between individual scans, our continuous scanning strategy significantly reduces scanning time and enables large areas to be scanned more efficiently than with conventional methods.For instance, the stop-and-stare approach typically requires ~11 minutes to obtain 1024×256×18 A-lines, including pose optimization and manual scanning point selection operation [22].In contrast, our continuous scanning strategy is capable of acquiring the same number of A-lines in ~30 seconds, resulting in a remarkable 14-fold increase in scanning speed at the same A-line rate.Moreover, the continuous movement of the robotic arm ensures uniform and seamless image acquisition, significantly eliminating the brightness variations and motion artifacts commonly found in traditional stop-and-stare methods, and enhancing the overall image quality.Furthermore, the strategy can be extended to accommodate larger or irregularly shaped objects by precisely controlling the robotic arm to reposition the scanner at various positions and angles.This adaptability enables customized scanning trajectories based on the unique shape and size of the sample, making the imaging process versatile across a wide range of applications, such as diagnosing and treating organs or tissues, as well as documenting and analyzing artifacts.In the revised manuscript, we have emphasized this novelty of our research in the Discussion section (line 421 -441).
Novel contribution 2: This is the first work aimed at inspecting and performing microsurgery for MSDs using robotic-OCT, which addresses an important problem in the field of digital forensics and has the potential to revolutionize the data recovery procedures by replacing conventional methods that involve destructive removal of entire insulating layers or the use of X-ray inspection techniques.Our proposed method can probe the internal multilayer structure of the MSD and accurately reveal the underlying PCB traces in a non-contact, non-destructive and fast manner, which eliminates the need for manual removal of the insulating layer.Utilizing low-power (a few milliwatts) near-infrared continuous wave light, the OCT imaging technique causes no harm to the operator, the device, or the stored data.It avoids the risks of harmful radiation, potential bit errors [25][26], and laser fault injection [27], while maintaining the integrity of the device.Importantly, our robotic-OCT system acquires high-resolution data consisting of 2000 × 3000 × 2048 voxels in ~ 37 seconds.This provides a significant advantage over micro-CT, which typically takes minutes to hours for scanning similar-sized areas [28].The detailed analysis of the insulating layer, distribution of pins and vias within an MSD, and the establishment of a comprehensive PCB trace database for various types of MSDs can be efficiently achieved through the obtained high-resolution OCT images.Furthermore, it can be utilized to identify any cracks, cuts, scratches, or burns in damaged devices, facilitating internal diagnosis of abnormal connections.This information is vital in determining the extent of damage, evaluating the circuit's integrity, and assessing the repairability of the device.Consequently, the non-destructive high-speed robotic-OCT imaging can maximally preserve the sample's integrity, greatly reduce manual labor, and significantly increase the chances of successful data recovery from the MSD.In the revised manuscript, we have emphasized this novelty of our research in the Discussion section (line 380 \ 400).
Novel contribution 3: Inspired by image-guided surgical interventions in the medical field, we have developed a robotic-OCT-guided laser ablation technique called the "Chip Surgery Robot".Robotic assistance ensures precise and controlled movements during the laser ablation process, while OCT provides high-resolution sub-surface imaging and guidance, along with quantitative information to evaluate the ablation process.This advanced technique enables precise microsurgery for MSDs by accurately and automatically removing unwanted layers or structures while minimizing sample damage.By doing so, it selectively exposes the relevant pins necessary for data recovery from the flash memory, eliminating the need to fully expose the entire PCB as done in conventional methods.As a result, this approach offers several advantages, including simplifying the subsequent welding process, and allowing operators to focus on specific areas without concerns about the rest of the sample or the risk of a short circuit.In the revised manuscript, we have emphasized this novelty of our research in the Discussion section (line 450 \ 460).
Although our work focuses on a specific application (data recovery from MSDs), it has the potential to be extended to various applications across different fields.For example, the proposed robotic-OCT-guided laser ablation technique can be utilized for precise and realtime identification of tumor margins during surgical procedures.Surgeons can accurately target and remove cancerous tissue while minimizing damage to healthy surrounding tissue.In the field of microelectronics, our technique's ability to precisely remove unwanted layers can be valuable for the fabrication of complex integrated circuits, where selective material removal is crucial for optimizing circuit performance.It also opens up new possibilities for broader applications in digital forensics, failure analysis, materials testing, and quality control, bringing advancements in precision and efficiency.In the revised manuscript, we have emphasized the application scope of our research in the Discussion section (line 473 \ 482).
Overall, our research provides innovative technological and engineering advancements in the inspection and microsurgery of MSDs, resulting in significant contributions to data recovery and reverse engineering.Additionally, our work not only addresses critical challenges in these fields but also holds potential for broader applications beyond our specific focus.Our work has also been recognized for its novelty by the Reviewer #2 and Reviewer #3.
2) Moreover, some storage devices have a large protective enclosure with a thick layer of plastic.Does the image quality change if a larger SD card has a thicker protective layer?

>GSQPOSG#
Indeed, the SD card have a large protective enclosure with a thick layer of plastic.However, it is important to note that this plastic casing serves only as an outer protective cover and can be easily removed, as shown in Suppl.In the revised manuscript, we have included supplementary material showcasing the inspection results of the SD card.Specifically, in Section 2.7 (line 369 -371), we have added the following description: For an SD card, the essential storage component inside the SD card was still an MSD, and the corresponding robotic-OCT inspection results were shown in Supplementary Figure S1.
?UQQM) 8KI) ?+) >PDPTKE(<5@ KOSQGETKPO PH CO ?6 5CRF) en face >GVKGWGR $, %>GNCRLS TP TJG 4UTJPR&3 General Comments: The manuscript titled "Robotic-OCT Guided Inspection and Microsurgery of Monolithic Storage Devices" presents a novel approach for non-destructive inspection and microsurgery of monolithic storage devices (MSDs) using a robotic-arm-assisted optical coherence tomography (robotic-OCT) system.The authors describe the capabilities of the system in rapid imaging, defect detection, pin identification, and laser ablation for targeted area removal.The manuscript highlights potential applications in digital forensics, failure analysis, materials testing, and quality control.Overall, the work is of significant novelty and addresses the need for non-destructive techniques in data recovery, and merits the publication.Nevertheless, there are a few points that require further clarification and improvement.

>GSQPOSG3
We thank the Reviewer for the positive comments and recommendation for publication of our manuscript.Please find below the answers to the specific questions.

Specific Comments: ABSTRACT
The abstract outlines the key features of the robotic-OCT system, including its ability to facilitate high-resolution imaging, PCB visualization, pin identification, defect detection, and selective removal of targeted areas using laser ablation.The abstract also highlights the diverse potential applications of the robotic-OCT technology in digital forensics, failure analysis, materials testing, and quality control.This is an interesting and very practical application topic.However, there are a few suggestions to enhance its clarity and completeness: 1) Quantify the benefits: Provide quantitative information on the benefits offered by the robotic-OCT system.For example, mention the achievable imaging resolution, the reduction in inspection time compared to traditional methods, and any improvements in data recovery efficiency observed in the study.Quantitative data will add credibility and demonstrate the practical advantages of the system.

>GSQPOSG3
Thank you for valuable suggestions.We have revised the abstract to include quantitative information on the benefits offered by the robotic-OCT system, including the achievable imaging resolution, the reduction in inspection time compared to traditional methods, and the improvements in data recovery efficiency observed in the study.All quantitative information in the following revised abstract is highlighted.
Abstract: Data recovery from monolithic storage devices (MSDs) is in high demand for legal or business purposes.However, the conventional data recovery methods are destructive, complicated, and time-consuming.We develop a robotic-arm-assisted optical coherence tomography (robotic-OCT) for non-destructive inspection of MSDs, offering ~7 F5 lateral resolution, ~4 F5 axial resolution and an adjustable field-of-view to accommodate various MSD sizes.Using a continuous scanning strategy, robotic-OCT achieves automated volumetric imaging of a micro-SD card in ~37 seconds, significantly faster than the traditional stop-and-stare scanning that typically takes tens of minutes.We also demonstrate the robotic-OCT-guided laser ablation as a microsurgical tool for targeted area removal with precision of 10 F5 and accuracy of ~50 F5, eliminating the need to remove the entire insulating layer and operator intervention, thus greatly improving the data recovery efficiency.This work has diverse potential applications in digital forensics, failure analysis, materials testing, and quality control.
2) Expand on the laser ablation technique: Discuss the precision and accuracy of the laser ablation process, potential challenges, and any considerations for ensuring minimal damage to the device during microsurgery.

>GSQPOSG3
We appreciate the valuable suggestions.We have revised the abstract to include the precision and accuracy of the laser ablation process as mentioned in the first comment.In Discussion section (line 460-469), we have also discussed the potential challenges and considerations for the laser ablation process to ensure minimal damage to the device during microsurgery.
It is important to note that the precision and accuracy of the laser ablation process determine the minimum pin size that can be accurately and reliably exposed.Our current further enhance the accuracy and precision, we can employ higher-resolution OCT systems and higher-precision robotic arms that can provide more detailed and precise guidance during the procedure, enabling better visualization of the target area and facilitating improved alignment of the laser beam with the pins.Meanwhile, it is essential to choose a laser power level that aligns with the specific diameter of the pin and the thickness of the insulating layer, which ensures the creation of suitably sized holes to facilitate subsequent processes such as welding and wiring during data recovery, while minimizing any potential damage to the internal circuitry.

Introduction
The Introduction section provides a comprehensive overview of the challenges and limitations associated with current methods of data recovery from monolithic storage devices (MSDs).It introduces optical coherence tomography (OCT) as a potential solution and highlights its advantages over X-ray radiography.The integration of a robotic arm and the use of continuous scanning are also mentioned as important components of the proposed approach.Overall, the Introduction sets the stage for the study and establishes the motivation for developing the robotic-OCT system.It is a high-quality introduction that deserves praise.However, here are a few suggestions to further improve the Introduction: 3) Address potential concerns: Given that OCT is traditionally used in the biomedical field, it may be beneficial to briefly address any potential concerns or challenges in applying OCT to non-biomedical applications such as MSD inspection.This could include factors like sample size, surface reflectivity, or other limitations that might affect the imaging quality or feasibility of the approach.
>GSQPOSG3 4) Clarify the novelty: Although the introduction mentions that this study is a significant improvement over traditional methods, explicitly emphasizing the novelty of this study can accurately highlight the research value more and contribution of the article and enhance the reader's awareness of the novelty and importance of the article.For example, the need and advantages of combining robotics with optical coherence tomography are emphasized.

Methods
The Methods section in this article is well-organized, detailed, and provides a comprehensive overview of the experimental procedures and techniques used to develop the custom-built robotic-OCT system.However, here are a few suggestions to further improve the Introduction: 5) It would be helpful to include more information about the specific components and manufacturers of the OCT engine, such as the brand and model of the super luminescent diode (SLD) and the k-linear spectrometer, OCT probe objective lens size, focal length, working distance.This additional detail will provide a clearer understanding of the setup and equipment used in the study.

>GSQPOSG3
praise and suggestion to provide more detailed information about the specific components and manufacturers of the OCT engine used in our study.
In the Methods section (line 494-506) of the revised manuscript, we have incorporated the brand and model of the super luminescent diode (SLD), the k-linear spectrometer, OCT probe objective lens size, focal length, and working distance.
OCT engine: We built a customized spectral-domain OCT engine (Fig. employed a super luminescent diode (SLD; IPSDW0825, InPhenix) as the light source centered at 850 nm with 105 nm -3dB spectral bandwidth.A custom-built k-linear spectrometer with a F2 prism (PS852, Thorlabs), a 1200 lines/mm diffraction grating (WP-1200/840-25.4,Wasatch Photonics), a fiber-coupled collimator (RC08APC-P01, Thorlabs) and an achromatic lens (#49-381, Edmund) was used to record the linear-in-wavenumber interferogram and eliminate the need for interpolation.The custom-built objective lens of the OCT probe has a diameter of 25.4 mm, a focal length of 25 mm and a working distance of 20 mm.The system's signal-to-noise ratio (SNR) was measured to be 110 dB, with 4.5 mW optical power at the sample and 8.3 -pixels 12-bit linescan CMOS camera (Octoplus, Teledyne e2v, UK), corresponding to 120 kHz A-line rate.We obtained B-scan images by fast-axis lateral scanning, with a duty cycle of 95%, containing 1000 A-lines, resulting in a B-scan frame rate of ~114 Hz.
6) It would be valuable to mention any safety measures or precautions taken during the laser ablation microsurgery procedure.This could include details about laser power settings, safety protocols, and any measures implemented to prevent damage to the sample or ensure operator safety.

>GSQPOSG3
Thank you for your valuable suggestion.In the Methods section (line 576-588) of the revised manuscript, we have included a paragraph specifically dedicated to discussing the safety precautions regarding laser power settings, safety protocols, and measures taken to prevent sample damage and ensure operator safety.
Safety precautions: First, all personnel operating the laser were required to wear safety glasses specifically designed for the laser's wavelength.This protected their eyes from potential injury caused by laser beams.Additionally, laser power settings were carefully adjusted within safe operating limits to achieve the desired ablation outcome while minimizing any potential risk to the sample or operator.Regular monitoring of the laser power output was conducted to maintain consistent and safe settings.Warning signs were prominently displayed, and unnecessary reflective surfaces were removed from the working area to prevent accidental reflections that could cause harm.Physical barriers and marked safety zones were established around the robotic arm to prevent accidental contact and enable immediate halting of the arm's operation in emergencies.Moreover, the robotic arm's posture was restricted to ensure that the laser beam was directed away from personnel and critical areas.Finally, comprehensive safety training was provided to all individuals involved, covering laser safety protocols, emergency procedures, and safe operation of the equipment.

>GSQPOSG3
We appreciate attention to detail and the opportunity to improve the quality of our work.Scale bars have been added to all relevant figures in the revised manuscript to provide a clear indication of the spatial dimensions.

Result
The results section clearly explains the details of the customized robotic-OCT system and the experimental procedure.Here are a few suggestions to further improve the Introduction: 8) Clarify the specific benefits and advantages of the continuous scanning strategy.How does this strategy improve the imaging process compared to other scanning methods?Highlighting these advantages and discussing any limitations or trade-offs would provide a more comprehensive evaluation of the approach.

>GSQPOSG3
We appreciate the reviewer's suggestions.We have included a more detailed discussion on the advantages and limitations of the continuous scanning strategy in the Discussion section (line 425-449) of the revised manuscript.
Unlike the conventional stop-and-stare scanning approach, our study introduces a novel continuous scanning strategy specifically designed for the robotic-OCT system, where the robotic arm moves continuously over the object and captures data.By eliminating the need for start-stop motions between individual scans, our continuous scanning strategy significantly reduces scanning time and enables large areas to be scanned more efficiently than with conventional methods.For instance, the stop-and-stare approach typically requires ~11 minutes to obtain 1024×256×18 A-lines, including pose optimization and manual scanning point selection operation [22].In contrast, our continuous scanning strategy is capable of acquiring the same number of A-lines in ~30 seconds, resulting in a remarkable 14-fold increase in scanning speed at the same A-line rate.Moreover, the continuous movement of the robotic arm ensures uniform and seamless image acquisition, significantly eliminating the brightness variations and motion artifacts commonly found in traditional stop-and-stare methods, and enhancing the overall image quality.Furthermore, the strategy can be extended to accommodate larger or irregularly shaped objects by precisely controlling the robotic arm to reposition the scanner at various positions and angles.This adaptability enables customized scanning trajectories based on the unique shape and size of the sample, making the imaging process versatile across a wide range of applications, such as diagnosing and treating organs or tissues, as well as documenting and analyzing artifacts.
Nevertheless, one significant limitation of the continuous scanning strategy is the management of the large amount of data generated during a single scan, especially when imaging larger objects.This can pose challenges in terms of data storage, processing, and analysis.Efficient data processing techniques and robust storage solutions are vital to effectively handle the increased data throughput.Another limitation is the accurate capture of geometric shape and surface details of objects to plan scanning trajectories, particularly for irregularly shaped ones.This limitation can be addressed by incorporating additional imaging technologies, such as a 3D camera, into the robotic-OCT system to provide accurate spatial information.9) Address the feasibility of extending the continuous scanning strategy to larger or irregularly shaped objects.Are there any practical limitations or challenges that need to be considered?Discussing the scalability and adaptability of the approach would contribute to the broader applicability of the robotic-OCT system.

>GSQPOSG3
We appreciate the reviewer's suggestions.Extending the continuous scanning strategy to larger or irregularly shaped objects is feasible and can be an advantage over the conventional scanning methods, but it indeed has practical limitations and challenges that need to be considered.We have discussed the adaptability of the continuous scanning strategy extended to larger or irregularly shaped objects in the Discussion section (line 437-449) of the revised manuscript.
Furthermore, the strategy can be extended to accommodate larger or irregularly shaped objects by precisely controlling the robotic arm to reposition the scanner at various positions and angles.This adaptability enables customized scanning trajectories based on the unique shape and size of the sample, making the imaging process versatile across a wide range of applications, such as diagnosing and treating organs or tissues, as well as documenting and analyzing artifacts.
Nevertheless, one significant limitation of the continuous scanning strategy is the management of the large amount of data generated during a single scan, especially when imaging larger objects.This can pose challenges in terms of data storage, processing, and analysis.Efficient data processing techniques and robust storage solutions are vital to effectively handle the increased data throughput.Another limitation is the accurate capture of geometric shape and surface details of objects to plan scanning trajectories, particularly for irregularly shaped ones.This limitation can be addressed by incorporating additional imaging technologies, such as a 3D camera, into the robotic-OCT system to provide accurate spatial information.
10) Additionally, it would be valuable to discuss the potential limitations or challenges encountered during the microsurgery procedure, such as the precision of pin exposure or any thermal effects caused by laser ablation.

>GSQPOSG3
This is a very important point.Indeed, as suggested by the reviewer, there are two potential challenges encountered during the microsurgery procedure.
One challenge is the precision of pin exposure.Accurate positioning of the robotic arm and alignment of the laser beam with the target area are critical to ensure precise pin exposure.Any deviations or misalignments may result in incomplete or inaccurate ablation, affecting the effectiveness of the procedure.In Section 2.6 of the revised manuscript (line 287-309), we described our experiments to assess the accuracy and precision of robotic-OCT guided laser ablation on target areas.
First, to assess the accuracy and precision of robotic-OCT guided laser ablation on target areas, we conducted experiments using a total of 18 micro-SD cards of the same model (Card 8).Each card underwent ablation on 10 different technological pins, with each pin considered as an individual trial.To evaluate accuracy, we compared the center position of each ablation hole with that of the corresponding pin in the OCT image.This allowed us to determine how closely the laser ablation process aligned with the intended target.For precision assessment, we compared the center position of each ablation hole with the average center position calculated from the 18 corresponding ablation holes in each trial.This analysis provided insights into the consistency and repeatability of the laser ablation This achieved precision and accuracy level is sufficient for majority of technological pin sizes.
Another challenge is managing the thermal effects caused by laser ablation.Laser energy can generate heat, which can potentially cause thermal damage to the surrounding materials.It is important to carefully control the laser power output to avoid excessive heating and minimize the risk of thermal damage.In Section 2.6 of the revised manuscript (line 310-335), we described our experiments to evaluate the influence of different power levels on the laser ablation process.
Secondly, we evaluated the influence of different power levels on the laser ablation process, as shown in Fig. 7.We selected a micro-SD card sample (Card 8) contained a 6×6 array of technological pins on its internal PCB, with each pin having a diameter o Following OCT guidance, laser ablation was performed on each row of pins in the micro-SD card for six power levels: 2 W, 6 W, 10 W, 14 W, 18 W, and 20 W, as shown in Fig. 7(a).By analyzing the en face OCT image of the card surface presented in Fig. 7(b), the diameters en face OCT image at the depth of 7c), the ablation holes appeared as bright spots and exhibited a progressive increase in size corresponding to the escalating power levels.Regarding ablation depth, demonstrated that both the size and depth of the ablation holes increased as the laser power level was raised.These results also indicated that the optimal laser power for this specific sample would be 10 W, as it would guarantee that the ablation hole size remained within the pin dimensions, and the ablation depth was close to, but did not exceed, the thickness of the insulating layer to prevent any potential damage to the PCB circuitry.This observation highlighted the importance of selecting an appropriate laser power level that strikes a balance between achieving the desired ablation results and minimizing excessive heating.
In the Discussion section (line 460-469), we have also discussed the potential considerations for the laser ablation process to ensure minimal damage to the device during microsurgery.
It is important to note that the precision and accuracy of the laser ablation process determine the minimum pin size that can be accurately and reliably exposed.Our current further enhance the accuracy and precision, we can employ higher-resolution OCT systems and higher-precision robotic arms that can provide more detailed and precise guidance during the procedure, enabling better visualization of the target area and facilitating improved alignment of the laser beam with the pins.Meanwhile, it is essential to choose a laser power level that aligns with the specific diameter of the pin and the thickness of the insulating layer, which ensures the creation of suitably sized holes to facilitate subsequent processes such as welding and data recovery, while any potential damage to the internal circuitry.

11) Discussion
The authors' discussion highlights the novelty and potential impact of their work on the field of data recovery from microsurgery for MSDs.They provide a clear contrast between the traditional methods of data recovery, which are time-consuming, require expertise, and can cause damage to the samples, and the proposed robotic-OCT system, which enables automated, non-destructive, and fast imaging.The authors rightly point out that their work has the potential to revolutionize data recovery procedures for MSDs.

>GSQPOSG3
Thank you for comments on the discussion section of our work.We appreciate recognition of the novelty and potential impact of our robotic-OCT system on data recovery from microsurgery for MSDs.

>GVKGWGR $-%>GNCRLS TP TJG 4UTJPR&3
The authors do a good job introducing the optical coherence tomography as a safe and fast option for inspecting the internal PCB layers of the monolith flash storage devices which can substitute the traditional X-ray inspection.The authors also automated the whole process which can help make forensic data extraction more efficient than before.This research can be a good reference for digital forensics labs to introduce OCT instead of X-ray for reverse-engineering the target device PCBs.Also, this work can contribute to make the data acquisition process quicker than before in digital forensics.

>GSQPOSG3
We would like to express our sincere appreciation to the reviewer for the recognition of our research and highlighting the impact of our work, as well as providing valuable suggestions to improve it.Please find below the answers to the specific questions.
1) However, one thing I feel missing is the detailed evaluation of the OCT inspection.
The authors do mention the limitation saying that the 2nd layer of the PCB cannot be observed due to the thickness of the epoxy.But up to how many micro meters can the OCT penetrate to observe further layers?If there are more than 2 layers of PCBs, then how many layers can be inspected?And does the material of the epoxy affect the efficiency of OCT inspection?Those details should be discussed in the article.

>GSQPOSG3
We appreciate comments regarding the detailed evaluation of the OCT inspection in our article.We would like to address concerns and provide further clarification.
The authors do mention the limitation saying that the 2nd layer of the PCB cannot be observed due to the thickness of the epoxy.But up to how many micro meters can the OCT penetrate to observe further layers?In the Discussion section of the original manuscript, we mentioned, "one of the primary limitations of OCT is the relatively low penetration depth, which makes it difficult to obtain an image of the PCB traces on the dice side because the plastic substrate is too thick."We apologize for the misunderstanding of this sentence and would like to clarify and provide further explanation regarding this statement.
As illustrated in Fig. 2(c), the micro-SD card structure comprises a PCB hosting electronic components, a protective plastic housing on the dice side of the PCB, and an insulating layer on the pinout side of the PCB.In our study, when imaging from the pinout side, we can only visualize a portion of the 2nd layer (dice side) of the PCB.This limitation is primarily attributed to the high reflectivity of the copper layer on the pinout side, rather than the thickness of the epoxy.Attempting to image the card from the dice side is not effective due to the light penetration being hindered by the thick plastic substrate.Consequently, no useful information can be obtained from this side.Since the majority of pertinent information for data recovery is concentrated on the pinout side, imaging the card from this side represents an optimized approach for our specific application.If there are more than 2 layers of PCBs, then how many layers can be inspected?In the case of MSDs, as illustrated in Fig. 2(c), they usually only consist of two layers: the pinout side and the dice side.It is uncommon to have additional layers beyond these two.Even if there were additional layers, the ability to inspect information from deeper layers would be further hindered due to the reflectivity of the copper layer.However, it is essential to emphasize that the majority of crucial information required for data recovery is typically concentrated within the first layer (pinout side).Therefore, the inability to observe deeper layers has minimal impact on the data recovery process.

And does the material of the epoxy affect the efficiency of OCT inspection?
The material used for the black insulating layer on the pinout side of micro-SD cards may vary among different manufacturers or even different product versions.However, we conducted OCT imaging on a diverse set of over 80 micro-SD cards, including different brands and models collected from multiple sources (see Supplementary Figure S2).Our observations consistently revealed that the insulating layers of these cards exhibited transparency to the near-infrared light used in our OCT systems.In Fig. 3, we presented representative OCT imaging results of four micro-SD cards, each with distinct brands, models, surface roughness, and insulating layer thicknesses.The B-scans clearly showed the presence of a black region within the insulating layer, indicating that no light was scattered from the inside of the layer.These results demonstrated that the material of the insulating layer did not significantly affect the OCT inspection process, as it was transparent to near-infrared light.We have incorporated these results into the Section 2.3 of the revised manuscript (line 193-199).
2) The authors suggest using the mid-infrared OCT, but no further detail is discussed.
Please make it clear what becomes better and what limitation the operators would face by changing the wavelength of the infrared.

>GSQPOSG#
3) Overall, the article lacks detailed explanation of each steps and investigations.For example, in section 2.5, the authors inspect variable damaged SD cards, but its results are pretty vague.For example, for the scratched card, how deep is the scratch based on the OCT inspection?Is it reaching the flash die and the data is not recoverable?Other sections are also a bit too much summarized.Please try to be more descriptive.The work itself is nice and helpful for forensic investigation.Since applying OCT in device inspection is the main part of this work (in addition to automating the laser ablation process), the readers would expect more evaluation details. 4) Also please reconsider the structure of the article.The whole time I was missing the background information (the one mentioned in section 4) and it was hard to follow all the sections.

>GSQPOSG#
Fig.S1(a)(b).When the plastic casing is removed, it can be seen that the essential storage component inside the SD card is still an MSD, as shown in Suppl.Fig.S1(c), allowing robotic-OCT inspection and data recovery to be performed without compromising the integrity of the device.Suppl.Fig.S1(d)shows the OCT image of the internal PCB traces of the MSD retrieved from the SD card.

Fig. 6
Fig. 6 The accuracy and precision of robotic-OCT guided laser ablation on target areas.(a) Absolute mean error for center positions of the ablation holes in the X (fast-axis) direction, indicating an accuracy of 52 .-(n=18samples per trial); (b) Center position distribution for the ablation holes in the X (fast-axis) direction, indicating a precision of ±10 .-(n=18samples per trial); (c) Absolute mean error for center positions of the ablation holes in the Y (slow-axis) direction, indicating an accuracy of 50 .-(n=18samples per trial); (d) Center position distribution for the ablation holes in the Y (slow-axis) direction, indicating a precision of ±11 .-(n=18samples per trial).For all box plots, center lines represent the median, the length of the box extends from the lower quartile to the upper quartile, whiskers are 1.5 times of interquartile range and red cross indicates outlier.

Fig. 6 .
Fig. 6.The accuracy and precision of robotic-OCT guided laser ablation on target areas.(a) Absolute mean error for center positions of the ablation holes in the X (fast-axis) direction, indicating an accuracy of 52 .-(n=18samples per trial); (b) Center position distribution for the ablation holes in the X (fast-axis) direction, indicating a precision of ±10 .-(n=18samples per trial); (c) Absolute mean error for center positions of the ablation holes in the Y (slow-axis) direction, indicating an accuracy of 50 .-(n=18samples per trial); (d) Center position distribution for the ablation holes in the Y (slow-axis) direction, indicating a precision of ±11 .-(n=18samples per trial).For all box plots, center lines represent the median, the length of the box extends from the lower quartile to the upper quartile, whiskers are 1.5 times of interquartile range and red cross indicates outlier.